Various types of yarn feed devices or feeders are known in which the yarn originating from a spool or bobbin is deposited onto a fixed drum loaded by an external member driven by its own motor, or onto a rotating drum from which it is withdrawn by the textile machine. In these feeders a system has necessarily to be provided for measuring or counting the number of turns present on the drum such that the yarn stock present on this latter remains virtually constant, and to prevent it from being totally consumed by the machine, with obvious problems for the operation thereof.
Various methods for measuring the yarn quantity (or number of turns) present on the drum are known. A first of these utilizes the reflection of light generated by an emitter and received by a corresponding receiver which are associated with the feeder. One or two reading zones (comprising emitters and receivers) are used to verify that at least one turn is present within them. Usually, one is positioned at the drum entry (yarn inlet zone) and one at the drum exit (yarn outlet zone) to control the so-called minimum stock and maximum stock respectively.
Feeders provided with this type of control are however able to ensure only that the number of turns is within a given range, but are not able to know their exact number (with the consequent impossibility of knowing how much yarn is stored on the drum, of which the lateral surface area is known).
The aforedescribed reflection method also has the limit of its well known dependence on the colour of the yarn to be monitored, and which can negatively affect the effectiveness of sensing the yarn by the optical elements utilized by the method under examination.
Feeders are also present in which the turns unloaded from the drum (and hence the fed yarn quantity) can be counted, again by reflection, however these known devices also present the limit that the reading resolution is strongly influenced by the yarn colour and by any dirt and dust deposits on the optical elements by which the number of turns is measured.
Other feed devices comprise optical elements inserted into a single emitter/receiver member and hence do not comprise separated emitter and receiver portions. This emitter/receiver member is of barrier operation and is able to measure the yarn quantity which has moved in front of it (i.e. the yarn quantity fed and hence the yarn quantity remaining on the drum), however as it does not know the exact position of the yarn within the sensor it is unable to know the yarn position at the feeder outlet, consequently it is unable to offer optimal resolution and precision.
Other feeders comprise mechanical solutions using mechanical lever detectors to which sensors (proximity sensors, Hall sensors) are connected to determine a minimum and a maximum yarn stock on the drum.
Such solutions again do not enable the number of turns present on the drum to be known exactly; moreover, the mechanical action of the levers modifies the yarn tension, with obvious repercussions on the yarn fed to the textile machine.